The present invention relates to channel allocation systems for cellular mobile communications systems covering several zones by using base stations for each zone, in particular, channel allocation systems for mobile communications systems that divide the above-mentioned zones into fan-shaped cell (sector cells) for each directional gain of sector antennas (antennas having different horizontal plane directivity), by contacting these sector antennas to each of the above-mentioned base stations.
Cellular mobile communications system such as car telephone systems repeatedly use the same wireless channels (channels) between zones that do not interfere with one another. Configuration of the above-mentioned zones in these mobile communications systems can be either omni zone configuration or sector zone configuration. In omni zone configuration, one antenna with no horizontal plane directivity (omni antenna) is installed at each base station to cover one zone centered around the base station supported by this omni antenna. With sector zone configuration, one base station is equipped with a number of sector antennas having directivity within fan-shaped horizontal planes and each of the above-mentioned sector antennas covers a fan-shaped area (sector cell) corresponding to directional gain of that antenna. In sector zone configuration, because there is little interference within the same channel because of the effect of the limited directivity of the sector antennas, repeat distance of the same channel is shorter than in omni zone configuration, and therefore there is a high rate of frequency (channel) use efficiency. The above-mentioned two methods for configuration of zones are covered in detail in the reference (Yoshikawa, Nomura, Watanabe, Nagatsu "Configuration methods for wireless zones for car telephones" Kenkyu jitsuyoka hokoku Vol. 23 No. 8, 1974). The above-mentioned mobile communications system also includes a number of fixed or mobile wireless terminals in the above-mentioned zones. These wireless terminals communicate with different wireless terminals or public communications nets via wireless channels (communication channels) between the wireless terminals and base stations of the zones to which they belong.
Allocation systems for communications channels of the above-mentioned mobile communications systems are classified as fixed channel allocation systems and dynamic channel allocation systems. Fixed channel allocation systems allocate channels for communications beforehand in a fixed way for each zone, taking into account mutual interference conditions between zones. The dynamic channel allocation system does not allocate channels in a fixed manner to each zone. When the base station sequentially selects a channel from among all channels for each communication request and certain allocation preconditions (communication quality) are satisfied at a channel, for example, the ratio between received power (desired wave power) of the signal from the other end of the communication link (one wireless terminal) and interference wave power (hereafter, this ratio is abbreviated as "CIR") exceeds certain threshold values for both uplink (transmission from wireless terminal, with base station the reception circuit) and downlink (transmission from base station, with the wireless terminal the reception circuit), the channel that satisfies those conditions is allocated as the communication channel. This dynamic channel allocation system makes effective use of channels through the collective use of all channels by all base stations. Because with this allocation system, the same channel can be re-used as long as the CIR threshold value is satisfied, the same channels can be re-used within shorter distances than with the fixed channel allocation system, making even more efficient use of channels. Thus, with the dynamic channel allocation system higher frequency (channel) usage efficiency than with the fixed channel allocation system is obtainable.
One conceivable way of obtaining high-frequency (channel) usage efficiency is to use a combination of sector cell zone configuration and dynamic channel allocation. As an example of a sector cell zone configured dynamic channel allocation system, there is an allocation system that prioritizes the same channels for sector cells in the same direction (Japanese Patent Laid-Open No. 081101 (1993)). Under actual propagation environments, at the downlink reception level, where the base station transmits and wireless terminals receive, there are locational variations due to interference by natural terrain or objects in the vicinity of the wireless terminals. It is known that, because this locational variation is strongly affected by topography and geographical features present in the arrival direction of the radio waves, the less of a difference there is between the arrival directions of the desired waves and interference waves, the higher the correlation of locational variation between the two. The reference (V. Graziano, "Propagation Correlations at 900 MHz", IEEE Trans. Veh. Technol. VT-27, No. 4, November 1978) covers this. Therefore, since when there is a small difference between arrival directions of desired and interference waves in downlinks, because the strong correlation between the two in regard to locational variation means that when wireless terminals move, there is a high probability that desired waves will become stronger, together with interference waves, and therefore, if there is a constant CIR threshold value for channel allocation, CIR for in-use downlinks becomes smaller, and the possibility of signal deterioration through interference is small. This system uses the above-mentioned allocation system in which sector cells in the same channel are prioritized.
However, just as the difference in arrival directions between desired and interference waves becomes smaller in the downlink in the system of allocation by the prioritization of sector cells in the same channel, there is a problem in that when channels are allocated, the effect of the correlation of locational variation between desired and interference waves cannot be adequately obtained. Below, an example of this is explained, referring to the conceptual diagram in FIG. 5.
The mobile communications system in FIG. 5 uses a sector zone configuration. A base station (BS) 11 is established in the first zone and base stations 12 and 13 are established in the second and third zones, respectively. This mobile communications system has many other zones as well, but, as explanations of these zones are unnecessary, they are not included in the figures. At the base station 11, a sector antennas 31 (31a, 31b, 31c, 31d, 31e and 31f), whose horizontal plane directivity has a 60.degree. half-value width angle (the angular width that encompasses the point where directional gain is just 3 dB smaller than directional gain in the central direction, where direction of maximum emission intensity is the center) are established. The sector antennas 31a, 31b, 31c, 31d, 31e and 31f are positioned in correspondence with the above-mentioned horizontal plane directivity to cover 6 equal sector cells 41 (41a, 41b, 41c, 41d, 41e and 41f), respectively around the base station 11. In the same way, the 6 sector antennas each of antennas 32 and 33 of the base stations 12 and 13 cover sector cells 42 and 43, respectively. A wireless terminal 21 is located in sector cell 41f, wireless terminal 22 is located in the sector cell 42f and the wireless terminal 23 is located in the sector cell 43a.
Let us now consider the case in which the base station 12 is currently using channel CH2 to communicate with the wireless terminal 22, the base station 13 is using channel CH3 to communicate with the wireless terminal 23 and the base station 11 allocates a new channel to the wireless terminal 21.
First, concerning the uplink, because neither wireless terminals 22 nor 23 are within the directivity direction of sector antenna 31f, which covers the wireless terminal 21, bother channels CH2 and CH3 satisfy CIR threshold value. Concerning downlink, the wireless terminal 21 is neither within the directivity direction of sector antenna 32f which covers the wireless terminal 22 nor within directivity direction of the sector antenna 33a which covers the wireless terminal 23, so CIR threshold value is satisfied and allocation is possible. However, although half-value width angle of the sector antennas is 60.degree., because radio waves of a certain degree of intensity are emitted in their environs, when channel CH2 is used in the downlink of the wireless terminal 21, interference waves from the base station 12 become a problem, as do interference waves from the base station 13 when channel CH3 is used. Therefore, in order to lessen the tendency of deterioration due to interference when wireless terminals are moved, it is desirable that channel selection for downlinks be conducted so as to minimize the differences between arrival directions of desired waves and interference waves.
At this time, there is a strong possibility that the channel allocation system by giving priority to the same channel for sector cells of the same direction will allocate channel CH2 which is used by the sector antenna 32f, whose directivity direction is the same as that of the sector antenna covers the terminal 21. In this case, channel CH3 is the channel in which difference between the arrival direction of desired wave and that of interference wave in the wireless terminal 21 downlink becomes smaller, but in the wireless terminal 21 the allocated channel is channel CH2, for which the difference between the arrival directions of desired wave and interference wave becomes greater.
The prior channel allocation system that the sector cells of the same direction are preferentially allocated involves a problem of not adequately obtaining the above-mentioned benefit in downlinks from the correlations of locational variation of desired waves to locational variation of interference waves.